The universal approach to development of a Plasmodium falciparum vaccine for human malaria has centered around stimulation of host immune response to dominant parasite surface antigens-hoping to interfere with invasion of host cells. After nearly two decades of intense research, with disappointing results, a 40 percent reduction in human mortality has just been reported from a recent trial of a synthetic vaccine based on this same strategy. Dr. Carson and co- workers feel that this solitary approach is unsatisfactory and that better understanding of parasite virulence is a prerequisite to intelligent disease cycle targeting and exploitation. Towards this end they have chosen the Babesia bovis model system, beginning with a gene probe (pK5) previously described as a putative marker for avirulence. They aim to begin to explore the genetics of avirulence in the interest of identifying a virulence factor(s) with immunogenic potential. Their objectives are: a) To determine the presence of pK5 in the genome of both virulent and avirulent B. bovis clones; b) to determine the full length pK5 gene sequence; c) to begin to characterize the pK5 gene product.